Raw crude oil generally contains undesirable impurities including inorganic and organic solids, salts, water droplets, unstable large polar molecules etc. which are the root causes for various fouling on processing equipment in refinery production. Equipment fouling is broadly defined as reduced production efficiency such as reduced throughput because of solid deposition on liquid transfer pipes and increased energy consumption because of reduced thermal transfer efficiency through thermal process walls. Equipment fouling due to the precipitation of the undesirable materials occur at various processing stages in petroleum refineries such as crude hot train exchanger, atmospheric towers, vacuum furnace and vacuum tower, coker furnaces, and hydro and thermal cracking units, results in substantial efficiency losses. It is desirable to remove these undesirable materials in crude oil before the crude oil is put through the subsequent thermal processes.
Inorganic salts typically include various metal chlorides, sulfides, and oxides etc. such as calcium, sodium and magnesium chlorides and other particulates. Salts cause corrosion in refinery systems that are expensive to repair and require more frequent shutdown and longer turn-around before profitable operation resumes. Corrosion is caused primarily by hydrochloric acid (produced from the hydrolysis of salts at high temperatures) in crude oil distillation columns and overhead systems. Since salts in crude oils are a significant problem and concern, removing such salts is an important operational process in a refinery.
Typically, desalting crude oil involves adding water to the incoming crude oil emulsifying the water and oil by shearing across a globe valve, which is also known as a mix-valve and allowing the oil and water to separate in a desalter settling vessel. The salt preferentially and fairly rapidly dissolves into the water immediately following the mix-valve so the remaining step is to separate the water from the oil. The oil and water are separated based on their density differences. Desalted crude exits from the top of the desalter settling vessel to the crude distillation tower while effluent water or brine exits from the bottom. However, desalting heavy crude oil in a refinery desalter system is challenging due to the relatively high viscosity of heavy crude and relatively high densities of heavy crude oil relative to the water with the captured salt. Moreover, water and oil emulsions for heavy crude oil tend to be more stable than for light oil and stable emulsions make desalting less successful or at least more difficult.
Because of the chemical incompatibility of crude oil, organic solids, and water, the separation of crude oil and water emulsions in many cases does not remove impurity solids into the water phase from crude oil. With extreme variation of chemical constituents of crude oils, there is not a universal demulsifier for crude oil/water emulsions to help provide for oil/water separation. Existing desalting processes are not only inefficient for removing undesirable impurities in crude oil, they may also create additional undesirable waste such as stable crude oil/water emulsions and increased solid and water content in crude oil. In addition, current desalting processes use a large amount of fresh water (>4% based on crude oil) and chemicals such as demulsifiers and wetting agents etc., such that the resulting water contains dissolved salts, oil droplets, and other organic solids. Disposing such contaminated water adds significant cost to the desalting process.
It has been known in the refinery industry that specific fouling problems such as those at atmospheric and coker furnaces can be mitigated by removing organic solid and inorganic solids in crude oil and feed heavy oil. However, there is not any practical process to remove such solids from crude oil. Even at a laboratory scale, removing such solids by filtration is not practically feasible because the solids in crude oil would clog up the filter quickly as the solids in crude oil exist in colloidal particles coated with sticky organic compounds. There is no known practical method to remove organic solids in crude oils.
Some crude oils contain large and polar compounds which are inherently unstable in the crude oils. When such crude oil comes into contact with the wall of processing equipment, such as in an atmospheric or coker furnace, those compounds tend to precipitate out forming a thermally insulating layer on the wall and resulting in a drastic reduction in thermal transfer efficiency. There is not any known practical process to remove these large unstable compounds to prevent such equipment fouling.
Any improvement to removing impurities from crude oil would be very desirable for refineries.